1. Field of the Invention
This invention relates generally to an automatic transportation system for transporting materials in factories, warehouse or the like by a linear induction motor-operated carriage movable along a track, and more particularly to such a transportation system incorporating means for sensing direction and speed of movement of the carriage.
2. Prior Art
There has been proposed an automatic transportation system for transporting materials for an assembly line in a factory which comprises a carriage propelled by a linear induction motor (hereinafter referred to as "LIM"). One such conventional system as shown in FIGS. 1 and 2 includes a pair of spaced rails 10 on a floor 12, and a plurality of stators 14 of LIM arranged on the floor 12 at regions spaced along the rails 10 for producing a traveling magnetic field, and a carriage 16 having wheels 18 disposed in rolling engagement with the rails 10, and a reaction plate 20 mounted on a lower surface of the carriage 16 which reaction plate serves as a secondary conductor of the LIM. The stators 14 are disposed between the pair of rails 10, so that the reaction plate 20 passes over the stators 14 during the travel of the carriage along the rails 10. A position sensor (not shown) is arranged adjacent to each of the stators 14 to detect the arrival of the carriage 16 at the stator 14 to produce a sensing signal. A controller (not shown) is responsive to the sensing signal to energize the stator 14 to apply a traveling magnetic field to the reaction plate 20 to propel the carriage 16 along the rails 10. The materials to be transported are loaded onto the carriage 16. When the carriage 16 passes over each stator 14, the carriage 16 is accelerated by the traveling magnetic field produced by the stator 14 and then is caused to move to the next stator 14 by inertia. Then, when the carriage 16 reaches this next stator 14, it is again accelerated. Thus, this procedure is repeated, so that the carriage 16 can be continuously moved along the rails 10. The carriage 16 is caused to stop at stations for loading and unloading purposes. Each station has the stator 14 which is energized by opposite-phase currents to apply a braking action to the carriage 16 to cause the carriage to stop at the station.
The conventional automatic transportation system further includes a speed sensor arranged at each station for sensing the speed of travel of the LIM-operated carriage 16. One example of such a speed sensor comprises a touch roller adapted to be brought into contact with either a side or an underside of the carriage for being rotated about an axis thereof, and a signal-generating device for producing a signal representative of the rotational speed of the touch roller. The signal-generating device comprises either a tachometer generator for producing voltage signal or a rotary encoder for producing pulses. With this conventional speed sensor, however, the travel speed of the carriage 16 can not be accurately detected due to a slippage between the carriage 16 and the touch roller and wear of the touch roller, since the touch roller is adapted to be brought into frictional engagement with the carriage 16.
Another example of the conventional speed sensor means comprises non-contact type sensors such as photosensors and a counter. More specifically, a pair of first and second noncontact sensors are disposed respectively at two points spaced from each other a predetermined distance along the rails 10. The counter starts a counting operation when the first position sensor senses the carriage 16 and then ceases the counting operation when the second position sensor senses the carriage 16. The speed of travel of the carriage 16 is determined in accordance with the time required for the carriage 16 to run between the pair of first and second sensors, that is, the contents of the counter. The conventional speed sensor means has also been found disadvantageous in that the start and termination of the counting operation are not effected accurately, so that the travel speed of the carriage 16 can not be accurately sensed.
When the carriage 16 is to be stopped at the station, the carriage 16 is decelerated sufficiently and then is brought into a stop position. Usually, when the carriage 16 is to be stopped at the station, the carriage 16 can not be brought exactly into the proper stop position smoothly, in which case the carriage 16 is slightly moved back and forth at a very low speed so as to be brought into the proper stop position. Thus, the speed sensor must sense such a low speed. In addition, the direction of movement of the carriage 16 must also be detected to bring the carriage into proper stop position. The conventional speed sensor means described above have not fully met these requirements.